This bug still exists on Chrome 43.0.2357.52. Firefox and IE11 are fine, AFAIK this is a Chrome only problem.
Repro steps: Fullscreen any Youtube video towards the end of a long topic (100+ posts). Like this one the OP posted above.
Then Esc or close the fullscreen. Chrome jumps back 5 to 20+ posts. As Riking says, the earliest loaded post. On a long thread this can be hundreds of posts.
If you try to fullscreen again, it will (sometimes) instantly exit the fullscreen, and jump posts as before. For me this only happens on a second or third try, could be any video on the page.
I'd add more examples but I can't find any here on Meta. Short topics don't have this problem, and most Youtubes here on Meta seem to be in the first couple of posts.
Plenty of examples on my Discourse instance if needed. One very long topic with tons of Youtube posts is found here.